Chemical-mechanical planarization or chemical-mechanical polishing, commonly abbreviated CMP, is a technique used in semiconductor fabrication for planarizing a semiconductor wafer or other substrate.
The process uses an abrasive and corrosive chemical slurry (commonly a colloid) in conjunction with a polishing pad and retaining ring, typically of a greater diameter than the wafer. The polishing pad and wafer are pressed together by a dynamic polishing head and held in place by a plastic retaining ring. The dynamic polishing head is rotated during polishing. This approach aids in removal of material and tends to even out any irregular topography, making the wafer flat or planar. This may be necessary in order to set up the wafer for the formation of additional circuit elements. For example, this might be necessary in order to bring the entire surface within the depth of field of a photolithography system, or to selectively remove material based on its position. Typical depth-of-field requirements are down to Angstrom levels for the latest sub-50 nanometer technology nodes.
The process of material removal is not simply that of abrasive scraping, like sandpaper on wood. The chemicals in the slurry also react with and/or weaken the material to be removed. The abrasive accelerates this weakening process and the polishing pad helps to wipe the reacted materials from the surface.
One problem in CMP is determining whether the polishing process is complete, e.g., whether a substrate layer has been planarized to a desired flatness or thickness, or when a desired amount of material has been removed. Over-polishing of a conductive layer or film leads to increased circuit resistance. On the other hand, under-polishing of a conductive layer may lead to electrical shorting. Variations in the initial thickness of the substrate layer, the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate can cause variations in the material removal rate. These variations cause variations in the time needed to reach the polishing end-point. Therefore, the polishing end-point often cannot be determined merely as a function of polishing time.
One way to determine the polishing end-point is to monitor polishing of a metal layer on a substrate in-situ, e.g., with optical or electrical sensors. One monitoring technique is to induce an eddy current in the metal layer with a magnetic field, and to detect changes in the magnetic flux as the metal layer is removed. The magnetic flux generated by the eddy current is in opposite direction to the excitation flux lines. This magnetic flux is proportional to the eddy current, which is proportional to the resistance of the metal layer, which is proportional to the layer thickness. Thus, a change in the metal layer thickness results in a change in the flux produced by the eddy current. This change in flux induces a change in current in the primary coil, which can be measured as change in impedance. Consequently, a change in coil impedance reflects a change in the metal layer thickness. However, a polishing pad may have to be altered to accommodate an eddy current measurement during real time polishing of a metal layer on a substrate.
Accordingly, in addition to advances in slurry technology, the polishing pad plays a significant role in increasingly complex CMP operations. However, additional improvements are needed in the evolution of CMP pad technology.